An active-matrix substrate for use in a liquid crystal display device and other devices includes switching elements such as thin-film transistors (which will be simply referred to herein as “TFTs”), each of which is provided for an associated one of pixels. As such switching elements, a TFT which uses an amorphous silicon film as its active layer (and will be hereinafter referred to as an “amorphous silicon TFT”) and a TFT which uses a polysilicon film as its active layer (and will be hereinafter referred to as a “polysilicon TFT”) have been used extensively.
Recently, people have proposed that a material other than amorphous silicon or polysilicon be used as a material for the active layer of a TFT. For example, Patent Document No. 1 discloses a liquid crystal display device, of which the TFT's active layer is formed out of an oxide semiconductor film of InGaZnO (that is an oxide made up of indium, gallium and zinc). Such a TFT will be hereinafter referred to as an “oxide semiconductor TFT”.
The oxide semiconductor TFT can operate at higher speeds than an amorphous silicon TFT. Also, such an oxide semiconductor film can be formed by a simpler process than a polysilicon film, and therefore, is applicable to even a device that needs to cover a large area. That is why an oxide semiconductor TFT has been used more and more often in a display device as an active element which achieves even higher performance in its switching operation and which can be fabricated with the number of manufacturing process steps and the manufacturing cost cut down.
In addition, since an oxide semiconductor has high electron mobility, even an oxide semiconductor TFT of a smaller size than a conventional amorphous silicon TFT could achieve performance that is equal to or higher than that of the amorphous silicon TFT. For that reason, by using oxide semiconductor TFTs, the area occupied by the TFT in a pixel region of a display device can be decreased, and therefore, the aperture ratio of the pixel can be increased. Consequently, a display operation can be performed with even higher luminance or the power dissipation can be reduced by decreasing the quantity of light emitted from a backlight.
For example, in a small-sized high-definition liquid crystal display device for use in a smart phone and other electronic devices, it is not easy to increase the aperture ratio of a pixel due to the limit on the minimum width of lines (i.e., the process rule). That is why if the aperture ratio of the pixel can be increased by using an oxide semiconductor TFT, a high-definition display operation can be carried out with the power dissipation cut down, which is advantageous.